To set the operating environment of a main RAM in an environment setting process without using a changeover switch by using a display storage means as a work area for execution of the environment setting process and setting the operating environment through execution of the environment setting process.

If it is decided that a specific key is pushed when a data processor 1 is started, a CPU 11 secures a graphic video RAM 19 as a work area and masks the RAM 19. The CPU 11 sets an operating environment on a CRT display device 31, uses a text video RAM 17 for display and also uses the RAM 17 as a work area. Then the CPU 11 stores the contents of the set operating environment in a nonvolatile memory device 21 and starts the processor 1 based on the operating environment.